This invention relates to an exhaust system for outboard motors and more particularly to an improved and compact silencing system for such motors.
As is well known, an outboard motor includes a silencing system for silencing the exhaust gases from its engine before they are discharged to the atmosphere. Because of the compact nature of an outboard motor, it is difficult to provide an effective silencing system that will silence all frequency ranges throughout the running of the engine. Normally, it is the practice to employ an expansion chamber that is formed within the drive shaft housing for performing a large portion of the silencing. It is also common practice to discharge the exhaust gases from this expansion chamber through an underwater discharge. In this way, the body of water in which the motor is operating serves to provide a portion of the silencing. However, the underwater discharge offers too much back pressure for the exhaust gases under idling and low speed operation. Therefore, most outboard motors further employ an above the water exhaust for such low speed operation. With the above the water exhaust, the silencing effect of the water is obviously lost. It has been difficult with previously proposed exhaust systems to provide effective silencing throughout the speed range during which the exhaust gases are discharged from the above the water exhaust.
It is, therefore, a principal object of this invention to provide an improved silencing for outboard motors.
It is another object of this invention to provide a silencing system for the low speed exhaust of an outboard motor.
It is a yet further object of this invention to provide an improved, compact exhaust system for outboard motors.
As has been noted, most outboard motors operate through a wide variety of speed ranges and hence the exhaust noise occurs over a wide range of frequencies that vary as the engine speed varies. Most silencing devices, however, are effective only within certain frequency ranges. Thus, it has been the practice to provide a number of silencing devices that operate in unison so as to effectively silence a wide variety of frequencies. However, with outboard motors, the compact nature frequently does not permit the use of such a wide variety of silencing devices.
It is, therefore, a further object of this invention to provide a silencing device that is effective over a wide range of frequencies.
It is a further object of this invention to provide a silencing device for an outboard motor that is automatically operative to silence varying frequencies of the engine noise.